The motor vehicle industry is actively working to develop alternative powertrain systems in an effort to improve vehicle fuel economy and reduce the level of pollutants exhausted into the air by conventional drivetrain systems equipped with internal combustion engines. Significant development efforts have been directed to electric and fuel-cell vehicles as well as hybrid hydraulic vehicles.
Unfortunately, these alternative drivetrain systems currently suffer from several limitations and, for all practical purposes, are still under development However, hybrid hydraulic and hybrid electric drivetrains have found success, particularly in vehicle applications with a lot of starting and stopping. Hydraulic drive systems facilitate conversion between mechanical energy (e.g., a mechanical output from an engine or a rotating shaft) and hydraulic energy (e.g., fluid pressure).
Hybrid hydraulic drive systems can generally be classified as series systems or parallel systems depending on the general arrangement of energy flow through the drivetrain system. One known application of a parallel hybrid hydraulic drive system is in a vehicle equipped with hydraulic launch assist (marketed by Eaton Corporation under the trademark HLA™). In conventional drive systems, frequent braking wastes a significant amount of energy as heat, especially in larger vehicles. Hybrid hydraulic drive systems capture this energy and subsequently release it at a selected time to assist the engine in launching the vehicle from the stopped position.
More particularly, when such a vehicle brakes, mechanical energy from the vehicle driveline is captured by the hydraulic drive system and stored in a high pressure accumulator as hydraulic energy. The stored hydraulic energy can then be converted back into mechanical energy by releasing the pressurized fluid stored in the high pressure accumulator. The mechanical energy can then be used to, for example, accelerate the vehicle or power other devices. However, current hydraulic drive systems are not as effective in improving fuel economy in applications where the vehicle operates at a consistent, steady-state speed. A regular gearbox, which creates a direct mechanical connection between the engine and the wheels, provides better efficiency and fuel economy during steady-state conditions.
Parallel hybrid systems address this issue through their ability to provide both a hydraulic and a mechanical power flow path through the system. However, the performance of a parallel hybrid system is also inherently limited by its dependence on a mechanical power path, which must provide multiple gear ratios to balance the power requirements of the vehicle with the capabilities of the engine. Essentially, the hybrid system is only able to add power to or subtract power from power that is transmitted through the traditional mechanical driveline.
Conversely, a series hybrid system can effectively decouple the power output of the engine from the power needs of the vehicle. In a series hybrid system, the main purpose of the engine is to replenish the supply of stored energy in the system. The stored energy is simultaneously used to provide energy to the vehicle as needed. The series configuration facilitates improved fuel economy by allow selection of engine operating points based on efficiency and other performance criteria instead of on the instantaneous energy needs of the vehicle. However, series hybrid systems lack a redundant mechanical power flow path through the system and therefore tend to be less efficient during periods of sustained vehicle speed (e.g., cruising).
Series hybrid systems are also perceived as less reliable than conventional mechanical power transmission systems and parallel hybrid systems due to the lack of a more traditional mechanical power transmission path. Series hybrid drivetrain systems are also more complex and more expensive due to the increased number and size of components needed to support the transfer of the full engine power range to the vehicle.
There is a desire for a hybrid hydraulic system that operates at high efficiency during both transient (e.g., start/stop) operation and steady-state operation while still having a simple and cost effective architecture